A linear displacement transducer of this type is shown in my previous U.S. Pat. No. 4,667,158 and is illustrated in FIG. 1. The transducer is a helical coil 2 of an electrical conductor wound at a uniform pitch on a cylindrical, thin-walled tube or bobbin 1 of an electrical insulator or a poor conductor such as stainless steel. Preferably, the tube has suitable properties for use as a dry bearing surface, for example Teflon. The helical coil 2 is fixed to the first of two relatively movable bodies for which the relative displacement is to be measured.
A nonferromagnetic, electrically conducting rod or preferably a tube forms a core 3 which is slidable within the bobbin 1. It is made, for example, of aluminum or copper and is fixed to the second of the two relatively moving bodies.
Preferably the coil is surrounded by a low and constant reluctance path so that changes in coil inductance with respect to core 3 movement is maximized. This is preferably accomplished by positioning a material, such as ferrite 5, having a high magnetic permeability, but low electrical conductivity surrounding the coil. This material provides the desired low magnetic reluctance which not permitting the formation of significant eddy currents and not exhibiting a substantial skin effect.
Preferably this high permeability, low conductivity material is itself surrounded with a tubular shield 4 of high electrical conductivity to confine the field of the coil to the ferrite 5 and the skin effect layer of the shield 4 and to prevent external fields from linking with the coil 2. The shield 4 confines the magnetic flux generated by the current in the coil 2 and shields it from stray fields over a wide frequency range. It is preferably made of a material having both high electrical conductivity and high magnetic permeability, such as soft iron or low carbon steel.
An AC electrical energy source 6 and a detector circuit means 7, preferably in the form of a bridge circuit, are electrically connected to the coil 2. The AC source 6 operates at a frequency, preferably in the range of 50-200 Khz, which may be designated a carrier frequency f.sub.c. An important key to the efficient and effective operation of a transducer of this type is that the frequency of the source 6 be high enough that the skin depth in the core 3 is substantially less than the radius of the core and less than the thickness of the wall of the tube.
The source 6 drives the coil through a resistor 8 which has a resistance which is much greater than the inductive reactance of the coil and its associated structures so that effectively the transducer is driven by a current source. Therefore, the voltage across the transducer coil 5 is approximately (V/R)*(2 pi f.sub.c L).
The detector circuit 7 detects a signal at an AM detector 9 which is proportional to the inductance of the coil 2 and its associated structures. The coil voltage is proportional to coil inductance, which in turn is proportional to the displacement of the core 3.
In the operation of the basic concept of the displacement measurement apparatus of FIG. 1, the AC source 6 excites the bridge circuit, including the transducer coil 2 in one of its branches. Because of the skin effect at the frequency at which the AC source 6 is operating, magnetic fields in the core 3 are confined to a thin layer approximately equal to the sum of the skin depth in the core material which is typically on the order of 0.25 millimeters thick plus the spacing from the exterior of the core 3 to the interior of the coil 2. Because the skin depth is considerably less than the radius of the core, the magnetic flux is confined to a path in the region of the core 3 which has a considerably smaller cross-sectional area than the flux path where there is no core 3. Since reluctance is inversely proportional to the cross-sectional area of the flux path, the core 3 has the effect of substantially increasing the reluctance and therefore substantially reducing the magnetic flux in the region of the core. With the core 3 partially inserted in the coil 2 of the transducer, the interior of the coil 2 can be divided into the region occupied by the core 3 where magnetic flux is low, and the region unoccupied by the core where magnetic flux is relatively high compared to the core region. Therefore, the flux linkages of the coil are substantially reduced as a result of the insertion of the core and are reduced in proportion of the extent of the insertion of the core within the coil 2. This, in turn, proportionally reduces the self inductance of the coil 2. Thus, the movable core varies the self inductance and the impedance and therefore varies the voltage across the transducer in proportion to its displacement.
While a great variety of detector circuits are known to those skilled in the art for detecting a signal which is proportional to the changes in coil inductance or voltage, the detector circuit of FIG. 1 operates well. A bridge is designed to be brought into AC amplitude balance by adjustable resistor 10 when the core 3 is centered within the coil 2. The AC source 6 is a signal at a frequency f.sub.c. The amplitude of the transducer signal at frequency f.sub.c at the node 11 of the bridge proportional to the displacement of the core 3. The amplitude of the balance signal at frequency f.sub.c at the opposite node 12 is adjusted so that it is equal to the amplitude of the transducer signal at node 11 when the core 3 is centered within the coil 2. A detector circuit means comprising two AM detectors 9A and 9B and a differential amplifier 14 are provided to detect the difference between the modulation amplitudes at the nodes 11 and 12.
The displacement of the core 3 is effectively providing an amplitude modulated signal at the terminal 11, the amplitude of which is proportional to displacement of the core 3 and may be detected by the AM detector 9B to provide an output signal which is directly proportional to the displacement of the core 3. The balance signal at node 12 is detected by an AM detector circuit 9A. The output signals from the two AM detectors 9A and 9B are applied to a differential amplifier 14, the output of which provides a signal V.sub.out which is proportional to the displacement of the core 3. Further details of the basic concept are described in more detail in my above cited U.S. Patent.
Embodiments of the invention, as described, can be applied or attached to relatively moving parts of substantially any machine. It is desirable to utilize such a device for detecting the displacement of a pneumatic or hydraulic cylinder such as used in control systems. Embodiments, as described above, can, of course, be attached externally in a variety of ways to such cylinders. However, exterior mounting would provide an additional, protruding structure on the cylinder which is unsightly, subject to damage, and would necessitate that additional space be allocated for it.
Therefore, the transducer is desirably positioned within the cylinder of the hydraulic or pneumatic actuator. In order for the transducer to be mounted in the actuator in a manner which does not interfere with the movement of the piston and its attached piston rod throughout its range of reciprocation within the cylinder, it is necessary that a hole be drilled into the piston/piston rod unit so that the transducer can be mounted on one end cap of the cylinder and can extend into the hole within the piston/piston rod. The transducer would extend completely into the bore in the piston when the piston is adjacent the end cap to which the transducer is connected.
The difficulty with using a transducer like the one illustrated in my above patent is that it would require a bore which is larger in diameter than manufacturers are willing to form in the piston/piston rod unit. Typically a maximum of 12 millimeters is permitted. Also, it would require that the slidable core be attached at the bottom of the bore to the piston/piston rod unit. Although the coil of the above embodiments of the invention are typically 6 to 10 millimeters in diameter, the addition of the externally surrounding ferrite and shield greatly increases the diameter of the transducer to typically on the order of 20 millimeters.
It is therefore an object and feature of the present invention to provide a displacement measuring transducer of the above type, but which can be mounted internally within such a cylinder in a considerably more efficient and less costly manner.
It is a further object and feature of the invention to provide a transducer of the above type which can have a coil diameter on the order of 6 to 10 millimeters, but not require a bore in the piston/piston rod unit of greater than 12 millimeters and to eliminate the need to attach a core at the bottom of the bore.